The shoulder joint is a ball-and-socket joint which has an exceptional range of motion. Shoulder instability and other maladies of the shoulder joint, such as arthrosis or fracture, may require a replacement of the joint.
A shoulder joint prosthesis which includes the replacement of the convex head of the humerus is disclosed in U.S. Pat. No. 7,819,923. It reflects the orientation and the center of rotation of actual joints. In the case of a worn or damaged rotator cuff or too much bone loss such a prosthesis would not recover the range of motion. In such a case a reverse shoulder prosthesis may be preferable. An example of such a prosthesis is disclosed in the U.S. patent application Publication No. 2009/0210065 A1. In such a prosthesis, the humeral component includes a stem and a cup attached to the stem. The glenoid component supports a convex head which articulates with the cup of the humeral component. For inserting the stem into the humerus a significant amount of bone substance has to be removed. This complicates later revision due to the low remaining bone volume.
An implant as disclosed in the U.S. patent application Publication No. 2009/0306782 A1 (WO 2007/041879) requires only a minimum of bone material to be removed. Here fixation of the implant within the bone is far less stable compared to a stem. EP 0578 345 A1 discloses an acetabular cup having an outer thread. CH 677 072 A5 discloses a further joint prosthesis with an outer thread.